The present invention relates to a method for the so-called reactive injection molding of a urethane-based resin composition and a polyol compound used therefor or, more particularly, to a method for the reactive injection molding of a urethane-based resin composition in which the shaped and cured article of the urethane-based resin composition can readily be released from the metal mold and a polyol compound used therefor with admixture of a specific silicone-based mold release agent to impart the cured urethane-based resin composition with good mold releasability.
It is a trend in recent years that the method of so-called reactive injection molding is applied to the molding of a urethane-based resin composition including polyurethane resins, polyurea resins, polyurethanepolyurea resins and the like. The reactive injection molding is a method in which at least two liquid components or slurry-like components with optional admixture of a filler having reactivity with each other to cause a curing reaction are separately ejected and brought into impingement so as to be rapidly and uniformly mixed together and the mixture is immediately injected into a metal mold where a shaped article is formed by the curing reaction taking place in the mixture. The metal mold is conventionally coated beforehand with a mold release agent such as waxes in order to facilitate releasing the cured and shaped article from the metal mold. This coating work of the metal mold with a mold release agent, however, requires a large amount of labor and a considerable length of working time to greatly decrease the productivity of the molding process. Accordingly, it is very desirable to omit the labor- and time-consuming mold release coating of the metal mold by admixing one or more of the reactive components with a mold release agent in advance or by use of a so-called internal mold release agent. Various internal mold release agents have been proposed for the reactive injection molding of urethane-based resin compositions including carboxyl-containing silicones disclosed in Japanese Patent Publication No. 55-1176 and hydroxy-containing organopolysiloxanes disclosed in Japanese Patent Kokai No. 59-56420.
These conventional internal mold release agents for urethane-based resin compositions are not quite satisfactory or disadvantageous in one or more respects. For example, the carboxyl-containing silicones sometimes deactivate the catalyst indispensable in the formulation of the urethane-based resin compositions so that the polyol component cannot be admixed in advance with the silicone and the molding machine for the reactive injection molding must be adaptable to three components. The hydroxy-containing organopolysiloxanes are usually poorly compatible with the polyol component in the formulation of the urethane-based resin composition so that the polyol component admixed with the organopolysiloxane must be thoroughly agitated directly before use in order to ensure full dispersion of the organopolysiloxane therein.